(1) Field of the Invention
The present invention relates to a method and device for controlling the phase of a timing signal and, more particularly, to a method and device for providing by digital processing a timing signal having a predetermined phase relationship with a timing signal component which is extracted from a received, modulated signal in a data modem and the like.
(2) Description of the Prior Art
When a signal modulated in phase shift keying (PSK) modulation or quadrature modulation is received to retrieve the original signal or data by demodulation, it is necessary to extract a timing signal component from the received signal and drive the demodulation circuit by a timing pulse signal having a certain phase relationship with the timing signal component.
Recently, demodulators are changing from the conventional analog circuit arrangement to the digital circuit arrangement, and in order for the digital circuit to perform the above-mentioned timing pulse signal phase control, the timing pulse signal of the digital circuit needs to match the sampled phase of the timing signal component.
In the conventional method for the timing signal phase control by a digital circuit, the timing signal component is extracted from the received, modulated signal and sampled at a certain sampling frequency, the phase of zero level or near-zero level of the thus sampled timing signal component is detected, and the phase of the timing pulse signal is controlled so that the timing pulse signal of the signal processing circuit is timed to the zero-cross point of the sampled timing signal component. In other words, phase control is performed such that the sampling point of time coincides with the zero-cross point of the extracted timing signal component.
However, in terms of the relation between the frequency of the modulated signal and the frequency of the timing pulse signal for the signal processing circuit constituting a demodulator, there are some cases where a certain phase difference must exist between the zero-cross point of the timing signal component and the sampling point. In dealing with these cases by the conventional method which matches the zero-cross point to the sampling point, it is necessary to use a very high sampling frequency or perform interpolation between sampling points. However, the digital processing circuit carries out the process at every sampling of data, and it is undesirable to increase the sampling frequency. This is particularly the case where a microprocessor with a limited processing speed is employed. Interpolation is also undesirable, since it requires much digital processing.